A gas turbine engine generally operates by pressurizing air in a compressor and mixing the air with fuel in a combustor. The air/fuel mixture is ignited and hot combustion gasses result, which flow downstream through a turbine section. The compressor typically includes compressor disks having airfoils dovetailed into the compressor disk. The compressor may include multiple disks, each having a plurality of airfoils.
Each of the compressor disk and the airfoils typically contain titanium, usually in the form of a titanium alloy. The titanium-to-titanium surface contact is susceptible to fretting wear and fretting fatigue. Fretting is the degradation of the surface usually resulting from localized adhesion between the contacting surfaces as the surfaces slide against each other. The problem of fretting is magnified in systems having a titanium-containing surface contacting another titanium-containing surface. For example, in a titanium compressor disk and titanium airfoil system, the fretting fatigue may result from movement of the dovetail of the airfoil within the slot in the compressor disk. As the disk rotates at a higher rotational speed, the centrifugal force on the airfoil urges the blade to move outward and slip along the surface of the dovetail. As the disk rotates at a lower rotational speed, the centrifugal force on the airfoil is less and the airfoil may slip inward toward the compressor disk. A second source of movement resulting in fretting fatigue in the dovetail system is the vibration from the airfoil. Aerodynamic forces may result in oscillation of the airfoil within the dovetail slot. The oscillation translates to high frequency vibration through the airfoil to the dovetail portion of the airfoil. As the airfoil vibrates, the surface of the dovetail section of the airfoil slides against the surface of the slot of the compressor disk, resulting in fretting fatigue.
In an attempt to solve the fretting wear and fatigue problem, the titanium dovetail surface of the airfoil may be shot-peened to create compressive stress in the airfoil surface. The increased compressive stress on the surface results in increased hardness, which reduces the adhesion between surfaces thereby reducing the fretting fatigue and wear. However, the shot-peening process requires expensive equipment additional processing steps and may result in surfaces having variability in roughness and dimensional accuracy. In addition, the shot-peened surface provides insufficient resistance to fretting fatigue and wear.
In another attempt to solve the fretting wear and fatigue problem, a coating of CuNiIn, aluminum bronze or a MoS2 lubricant may be coated onto the airfoil's dovetail surface to provide a surface that experiences less adhesion between surfaces. The application of lubricants such as MoS2 provides some protection from localized adhesion initially, but lubricants and lubricant coating wear away or deteriorate under service conditions for a gas turbine engine. The reduced adhesion acts to reduce fretting fatigue and wear, but does not provide reduced adhesion throughout the operational conditions of the compressor disk/airfoil system. The conventional lubricant coatings also eventually lead to material transfer between the surfaces. In addition, the coated dovetail surface provides insufficient resistance to fretting fatigue and wear.
Carburizing is a method that has been used to increase hardness of a surface. It is a well-known method for hardening steel surface to improve wear properties. Known carburizing methods take place at high temperatures, including temperatures of greater than about 1700° F. (927° C.). High temperature carburization methods suffer from the drawback that the method requires expensive, specialized equipment, capable of operating under high temperatures. Thermal treatments of blade dovetails and disks preclude use of conventional carburizing practices.
What is needed is an inexpensive, low-temperature titanium treatment that reduces fretting fatigue and wear that does not suffer from the drawbacks of the prior art.